The present invention relates to an image pickup apparatus using a solid-state image sensor in which two lines are simultateously scanned, and more particularly, relates to an image pickup apparatus which outputs a still picture stopping one moment of a picture in motion and a zooming-in picture with an area of a picture enlarged by electronic means.
Video cameras and VTRs (video tape recorders) have become increasingly compact in size and light in weight, and recently their prevalence has been rapidly growing in the form of a so-called video camera combined with VTR.
In such a video camera combined with VTR, a camera tube was initially used as an image sensor, but that has been replaced by a solid-state image sensor which applies the LSI technology which is characterized by compactness, light weight, high reliability, high image quality, etc., so that the solid-state image sensor is presently taking the lead.
A type of the above mentioned solid-state image sensor is an MOS-type sensor in which two lines are simultaneously scanned. FIG. 16 shows a schematic illustration of said solid-state image sensor.
In the solid-state image sensor, photoelectric conversion elements, e.g. photodiodes, are lining up on a light receiving surface in the shape of a matrix, and moreover, color filters are placed in order on the photodiodes. Since one photodiode is equivalent to one picture element, the photodiode will be hereinafter called the picture element.
FIG. 16 is a schematic illustration showing one example of the array of picture elements in an MOS image sensor, in which example all color transmission (namely, transparent) filters W, green transimission filters G, cyan transmission filters C.sub.y, and yellow transmission filters Y.sub.e are placed in order.
When video signals are read out from the image sensor, the charges, stored in the picture elements, of the (2m-1)-th line and the 2m-th line are read out in turn from left in a first field, and the charges of the 2m-th line and (2m+1)-th line are simulataneously read out in a second field. This is called "two-line simultaneous scan."
The signals corresponding with the individual color filters which have been read out by the above scan are called signal W, signal G, signal C.sub.y, and signal Y.sub.e. These signals, respectively, are outputted from independent terminals. The outputted signals W, G, C.sub.y, and Y.sub.e are added or subtracted at a certain ratio, resulting in the production of chrominance signals (for example, a red signal R and a blue signal B) and a luminance signal Y. For example, signal Y is obtained by adding the signals W, G, C.sub.y, and Y.sub.e at a ratio of about 1:1:1:1 as shown in the following formula: EQU Y=(W+G+C.sub.y +Y.sub.e) - - - (1)
Herein W=R+G+B, C.sub.y =G+B, and Y.sub.e =G+R, so that the formula (1) becomes as follows: EQU Y=8{1/4R+1/2G+1/4B} - - - (2)
In NTSC formula the luminance signal Y has the following composition ratio: EQU R:G:B=0.3:0.59:0.11 - - - (3)
whereas the ratio of mixture in the formula (2) is different from the foregoing ratio. However, since the sensitivity to the blue light (B) in the solid-state image sensor is lower than that to the red and green lights (R) and (G), in reality the ratio of mixture in the formula (2) becomes nearly equal to that in NTSC formula. Thus, the formula (2) leads to the production of the luminance signal Y which is almost equal to the luminance signal of NTSC formula.
As a result, the color and luminance signals required for the production of video signals can be obtained from the solid-state image sensor in which two lines are simultaneously scanned.
Now, using two picture element signals of the same line, consideration is made of the signals which are represented by the following formulae: EQU Y.sub.1 =(W+G) - - - (4) EQU Y.sub.2 =(C.sub.y +Y.sub.e) - - - (5)
In the same manner as in the above description, the following formulae are obtained by substituting W=R+G+B, C.sub.y =G+B, and Y.sub.e =G+R: EQU Y.sub.1 =4{1/4R+1/2G+1/4B} - - - (6) EQU Y.sub.2 =4{1/4R+1/2G+1/4B} - - - (7)
These are the same as the formula (2). This indicates that each individual luminance signal can be produced from the signal of each line. That is to say, scanning lines twice as many as the usual ones can be obtained in a vertical direction. It is called "two-line luminance signal independent processing" that two liminance signals are obtained by processing two individual signals independently as described above. The examples using said two-line luminance signal independent processing include Japanese Patent Application Laid-Open Nos. 59-50684 and 58-173989.
In accordance with the arts disclosed in the above mentioned documents, a recording means to record the luminance signals Y.sub.1 and Y.sub.2 is provided, and the odd-numbered line luminance signal Y.sub.1 and the even-numbered line luminance signal Y.sub.2 recorded in the recording means are changed by field and then outputted, thus enabling a frame picture to be reproduced from the information at the same time during one field period. As a result, there occurs no double image in a frame still picture, which may be caused when imaged with an ordinary camera, and a frame still picture of high image quality can be obtained.
In recent years, furthermore, semiconductor technology has markedly progressed, coupled with the further enlarged capacity and lowered cost of memories as well as the development of high speed A/D and D/A converters for processing of pictures. This has enabled semiconductor video memories to be used for civil-use appliances. In case of the arts disclosed in the above mentioned gazettes as well, frame pictures of high image quality can be easily obtained by using the semiconductor memories as recording means.
In the future, moreover, semiconductor video memories are expected to be used for various purposes in video cameras.
One of the applications of said memories is the so-called electronic zooming-in, which performs zooming-in not by optical means, but by electronic means. Namely, when an area of a picture is once recorded in a video memory and is again read out from the memory, the area is enlarged (by two times assuming that the frequency is 1/2) on the screen of a cathode ray tube (CRT) by reading out at a lower frequency (for example, 1/2) than the write frequency in both horizonal and vertical directions.